1. Field of the Invention
The present invention relates to a joined structure of different metals which has improved corrosion resistances at the joined interface. The present invention also relates to a friction welding method that is desirably used for manufacturing the above-mentioned joined structure of different metals.
2. Description of the Related Art
With respect to joints formed by joining members of different metals through a metallurgical method such as a friction welding method, conventionally, a joint which has a flange that is caused by a material melted or discharged during the joining process and bent toward the side of the base material of the flange with respect to the joined interface, or a joint on which the flange is ground to provide a smooth circumference of the joined interface, has been used.
However, the respective metals (pure metals or alloys) have natural electric potentials that are inherent to the components, and when different metals are placed very close to each other or in a joined state and an electrolytic solution is supplied between the different metals, a phenomenon similar to that in a battery takes place due to the difference between the natural electric potentials, causing corrosion to progress between the different metals. For this reason, in the case of the conventional joints in which the circumference of the joined interface is continually exposed to the external atmosphere, when the surface is not coated with paint, etc., or when the coated paint, etc., has separated therefrom, an electrolytic solution, which is a main source of corrosion, is easily supplied to the joined interface, resulting in failure to prevent the progress of corrosion.
The present invention has been devised so as to solve the above-mentioned problems, and objects thereof are to provide a joined structure of different metals which can improve the corrosion resistance at the joined interface by utilizing a flange that is generated during a joining process and which is usable even in severely corrosive environments such as the use in a location susceptible to salt damage.
The joined structure of different metals of the present invention, which is a joined structure formed by joining members of different metals, is characterized in that a flange is allowed to extend in a direction from the circumferential side of one of the members along the circumference of the other member. In the present invention, a flange refers to an area stretching outside from a line connecting the end portion of the original joining member at the time of joining is performed.
As shown in FIG. 1, in accordance with the joined structure of different metals of the present invention, a structure in which this flange 1 covers the circumferential portion of the joined interface forms is formed by allowing a flange 1 to extend in a direction from the circumferential side of one of members (first member) 2 along the circumference of the other member (second member) 3. In this structure, it is possible to prevent an electrolytic solution such as salt water from reaching the joined interface by the existence of the flange. Moreover, even in the case when the electrolytic solution enters the inside of the flange 1, the flange 1 or one portion of the second member 3 in the vicinity of the joined interface is subject to corrosion by this electrolytic solution to generate an oxide product 4, and this oxide product 4 accumulates in a gap between the flange 1 and the second member 3 so that a further supply of the electrolytic solution to the vicinity of the joined interface is blocked, thereby inhibiting corrosion at the joined interface. Therefore, the joined structure of different metals of the present invention has an effect for delaying the progression of corrosion at the joined interface as described above, thereby achieving improvement of corrosion resistances.
In another aspect of the joined structure of different metals of the present invention, by taking the natural electric potential difference between the different metals into consideration, the flange that is extended in a direction from the circumferential side of one of the members along the circumference of the other member is made of a metal that is relatively low in natural electric potential between the different metals. In other words, as shown in FIG. 2, in a joined structure having members of different metals being joined to each other, a flange 1, which is made of a metal member that is relatively low in natural electric potential, is formed so as to extend in a direction from the circumferential side of a metal member 5 having a lower natural electric potential along the circumference of a metal member 6 having a higher natural electric potential so that the metal member 5 having a lower natural electric potential is arranged so as to cover the circumferential portion of the joined interface.
In this aspect, when an electrolytic solution is supplied between the flange 1 made of a metal member having a lower natural electric potential and the metal member 6 having a higher natural electric potential, the area of the flange 1 is subjected to corrosion. The corrosion of this flange area serves as a sacrifice corrosion for the corrosion at the joined interface. In other words, the electrolytic solution that will cause corrosion at the joined interface actually uses for causing corrosion between the flange 1 and the metal member 6 having a higher natural electric potential so as to decrease in the corrosion at the joined interface. Therefore, in this more preferable embodiment of the joined structure of different metals of the present invention, in addition to the above-mentioned effect of the flange for preventing the electrolytic solution from reaching the joined interface and the effect of the oxide product for blocking corrosion, the sacrifice corrosion effect of the flange is exerted so that it becomes possible to further improve the corrosion resistance of the joined structure of different metals.
The joined structure of different metals having such a joined interface is formed by, for example, a friction welding process that is one type of solid-phase joining method. In the friction process, the surfaces of the joined members are mechanically cleaned, and-in the succeeding upset process, a reaction product generated at the joined interface is externally discharged, and the press-welding process between the two joined members is completed. Here, the member, which has been discharged together with the reaction product, forms a flange.
The friction welding method of members of different metals of the present invention is characterized in that a friction pressure is not more than the proof stress of the member having a lower melting point at the frictional interface temperature, and an upset pressure is not less than the proof stress of the member having a lower melting point at normal temperature. With this friction welding method, in a joined structure having members of different metals joined to each other, it becomes possible to allow a flange to desirably extend in a direction from the circumferential side of one of the members along the circumference of the other member.
In the friction process in the friction welding method of the present invention, the friction pressure must be set to not more than the proof stress of the member having a lower melting point in the friction interface temperature in order to store heat in an axis portion without continuously discharging the flange. This friction pressure is determined by the composition of the member having a lower melting point, and more specifically, the proof stress in a joined structure between a steel product and an aluminum alloy at an interface temperature of 450xc2x0 C. is set to be 17 MPa in the case of an aluminum alloy (JIS A5052-H34), and 22 MPa in the case of another aluminum alloy (JIS A5454). Moreover, in the upset process, it is necessary to set the upset pressure to be not less than the proof stress of the member having a lower melting point at normal temperature in order to bend the axis so as to allow the flange to cover the joined interface. In the same manner as the friction pressure, this upset pressure is also determined by the composition of the member having a lower melting point, and more specifically, the proof stress at a normal temperature of 25xc2x0 C. in a joined structure between a steel product and an aluminum alloy is set to be 215 MPa in the case of an aluminum alloy (JIS A5052-H34), and 240 MPa in the case of another aluminum alloy (JIS A5454).
Moreover, in the friction welding method of the present invention, when a friction time in the friction process is insufficient, the joined face is not sufficiently cleaned, that is, stains and residual oxides are excessively left at the joined face, causing a failure in obtaining a better welding state in the subsequent upset process. In contrast, when the friction time is too long, although the joined face is sufficiently cleaned, the input quantity of heat to be supplied to the joining members becomes too great, resulting in an excessive growth of the reaction product layer in the upset process and the subsequent serious degradation in the joining strength. For this reason, in the present invention, it is preferable to set the friction time appropriately by taking the above-mentioned friction pressure and upset pressure into consideration. Moreover, with respect to the upset time, any period of time may be set as long as it provides a sufficient press-welding process of the joining members.